1. Field of the Invention
The present invention relates to a magnetic memory device. In particular, the present invention relates to a magnetic memory device using a magnetic tunnel junction.
2. Description of the Background Art
A structure in which an insulator is sandwiched between two ferromagnetic materials is called “magnetic tunnel junction (MTJ)”.
In this structure, when a current tunneling the insulator is measured, it is observed that the measured current value varies depending on the directions of magnetization of the two ferromagnetic layers.
This phenomenon is called “tunnel magnetic resistance (TMR) effect”. The TMR effect is disclosed in detail in Japanese Patent Application Laid-Open No. 2002-231904.
A device for storing data in use of the above TMR effect, which makes correspondence between the magnetization directions of two ferromagnetic layers and two values, “0” and “1”, is an “MRAM (Magnetic Random Access Memory)”.
An MRAM has a structure in which magnetization directions of ferromagnetic layers are controlled by generating a magnetic field in use of current flowing through write wires, that is, bit lines and digit lines. The bit lines are arranged above the upper ferromagnetic layer and the digit lines are arranged below the lower ferromagnetic layer. Those lines are provided so as to intersect each other in a planer view.
A conventional MRAM has a structure in which an MTJ element is arranged as close to a bit line as possible. For example, as shown in FIG. 1 of Japanese Patent Application Laid-Open No. 2003-86773, generally, MTJ elements are directly brought into contact with lower faces of bit lines.
In the case where the MTJ element is directly brought into contact with the lower face of the bit line, as described above, a difference is generated between current density of the bit line and current density of the digit line required for writing data to MTJ elements. As a result, large difference between operating lives of each write wire may occur, so that there has been a problem that it is difficult to ensure reliability for both types of write wires.